No sound when on, then suddenly sound

[aro]

After years of playing very little guitar and not doing any amp tweaks or builds, I finally got back into it. The problems appeared promptly (well, maybe not promptly, maybe exactly because of the long break). I have this Marshall JCM 800 2204 clone, which I didn't turn on more than 5 minutes in the last few years. A few week ago I started playing it again, and it was all good (with some weird static once in a while, which would go away quickly).

Yesterday evening, as I started it, or maybe during playing, can't remember, it started squealing constantly, and no guitar sound came through. I turned it off, waited a little, back on, and no squealing, but also no sound at all. Totally quiet. Turned it off, waited, on, and then some constant loud static.

This morning I opened it up and first thing I checked the continuity in the V1, switches and jacks area. All seems to be good. I started playing it and it was all good. A little static at some point, but it went away. I started swapping V1 tubes, and I was doing this by turning it off and on.

At some point, as I turned it on, there was not guitar sound at all. Turned it off, waited, on, no sound. Off, wait, on, now I have sound.

One more time, turned it off, on, no sound. As I was strumming the strings, with no sound, the sound suddenly came on.

What can this be? The filter caps?

[shooter]

What can this be? The filter caps

easy enough to check...IF.. you're safe about it.  gator-clip your meter on the power supply, bring it up and watch.

You sure it's amp and not guitar or cables, have they been setting also?

[aro]

I tried different guitars and cables.

I just turned it on and the voltage on both caps is 440V. I had guitar sound, but it was popping and scratching, even with the pre-amp and volume on zero.

[tubenit]

When I have been playing and then suddenly get squealing and then no sound ................ and then the amp will play again after it was turned off for a while but then does the same squealing and no sound ...........................

This has always been an oscillation issue for me.  Sometimes you can figure this out by playing at a lower volume softly and the amp sounds OK,  but then you play louder and more aggressively, it squeals.  Oscillation can occur from the power tube "redplating".

It has always proved to be a bad tube and usually power tube.  The issue is what made the tube go bad?  Is it biased wrong?  Is it just an old tube gone bad?  I've had both old and new tubes suddenly go bad.

Checking voltages carefully and safely on plates, screens and cathodes of the power tubes can often give you a clue.  Also look and see if there is anything obviously amiss on the circuit board such as a broken wire or burnt resistor, etc..........

Also you will need to know your bias voltage and whether it is in range.  IF the power tubes grid resistor is broken or not grounded that can be the issue?

Sometimes just replacing the power tubes with new ones could resolve it?   However, I would like to know what made the tube go bad?

With respect, Tubenit

[tubenit]

Now having said all of the above (post)................

I have played an amp and had the sound come and go with OUT the squealing.  That's usually been an issue with input jacks, effects loop jacks, and or a broken connection or ground. On rare occasion, it's been a tube socket needing replacing or fixed.  Those can all cause popping and static stuff too.

With respect,  Tubenit

[aro]

On rare occasion, it's been a tube socket needing replacing or fixed.  Those can all cause popping and static stuff too.

Tubenit, thank you, sir. I looks like this might be the problem.

After seeing that I have pops and static with volume and gain on zero, even a hack like me realized it has to be something in the power section. After swapping new tubes, old tubes, tubes amongst themselves, I got one step further.

The cathode current on one of the sockets keeps jumping around, regardless of tube. This is further confirmed when I pull them out. The constant one is very hot, the jumpy one is just warm.

Does this point to a faulty socket? If yes, should I just replace it, or try some sort of repair?

[shooter]

try some sort of repair?

I'd hit all the connections on that socket with my iron and new solder, which if you replaced it would give the same results.   Look really close for carbon between pins, inside pin cups, re-tension the socket.  Sounds like you're there......for this problem!

[aro]

I tried to re-tension, but can't figure it out. Preamp sockets are pretty obvious, but not those power tubes.

[aro]

Well, I just did something that I think was retensioning, and so far, all is good. No noise, the current remains constant and they are equally hot. I'll keep an eye of them for a while, but maybe that was it.

[shooter]

Sounds like you got it, a few hours of play and bouncing around the amp should confirm that

[aro]

Now I really messed up somehow. There still was some static after retensioning, much less than before, but still... Also, turning it off would kick in the static and then it would die off. I touched up the solder joints lightly, and it seemed to be good. Voltage and cathode current were stable.

Then I touched heater pin 2 with the probe. Bright white sizzle  Turned it off right away, and eventually back on. No sound. One filter cap measured OK, the other one was in the mV range. Well... I'm gonna get one and replace it, and then see what happens.... I don't understand why the sizzle. I was very careful not to bridge two pins with the probe (something I have done years ago and now I'm obsessive about).

[Ritchie200]

Not sure about the sizzle...


However, does this build have an effects loop?  If so, check the grounding/bypass lug on the send jack and clean and retention if needed.  I see this all the time on amps that have been "sitting around unused".


Good luck!
Jim

[aro]

I did put in an FX loop years ago. I don't think it's related though. It has a on/off switch, which bypasses it entirely, and all those problems were with it off.

[tubenit]

I touched heater pin 2 with the probe. Bright white sizzle

Unfortunately, I think you touched pin 2 (the 6.3v AC)  to the pin 3  high voltage which is DC.   

You need to see if the heater voltage is still OK.  IF that is OK, then try your power tubes in another amp if you have one to make
sure they are OK also.

With respect, Tubenit

[aro]

You need to see if the heater voltage is still OK

The heater voltages are in the mV range. Did I blow up the PT?

Unfortunately, I think you touched pin 2 (the 6.3v AC)  to the pin 3  high voltage which is DC.

I could swear I didn't... I checked (unplugged), and the pin of the probe is too thin to from a bridge, you have to keep it at angle to touch the neighboring pin. I remember coming straight down with the probe, not at an angle. You have to be quite sloppy to bridge them (like I was years ago). I don't see what else could have happened though... I think I remember though just touching pin 2 with the sharp tip and off it went.

Now I'm waiting for the capacitor to arrive. Too bad you can't walk into a RadioShack and get one. Does it look like I need a PT?

[tubenit]

Let's make sure your voltmeter is not damaged.

Put in a fresh battery and measure known voltages such as 110VAC outlet plug or a 9VDC battery. 

Assuming the voltmeter is accurate.  Check voltages carefully safely for 6.3vac.  Are you getting less than 1 volt?  IF so, then you will need to buy a 6.3v transformer.  They are not expensive. 

However, also check the high voltage on the rectifier.  Are you getting good readings?  IF not, your PT is probably damaged.  IF so, then you can probably use your PT and the HV is fine.

Please let us know what you find out.   Sorry you are having so much trouble with this amp. Stick with it, you'll get it fixed.  I accidently destroyed a PT on the first tube amp that I worked on.  Stuff like that happens.  Get over it and buy the replacement parts move on.

With respect, Tubenit

[aro]

Voltmeter is good. I checked both a battery and wall plug.

Assuming the voltmeter is accurate.  Check voltages carefully safely for 6.3vac.  Are you getting less than 1 volt?  IF so, then you will need to buy a 6.3v transformer.  They are not expensive.

Sorry, but I don't know what you mean by this. Where/how should I check 6.3V AC?

Rectifier is good; 335V AC coming from PT.

Thank your for your help.

[shooter]

If it was me, I'd pull all the tubes and just measure 6.3VAC at the sockets.  If you get 6.3Vac, power-down, install the pre-amp tubes and measure the 6.3Vac in the PA tube sockets.  If it's still 6.3, power-down, install the PA tubes and measure on the bottom of a pre tube socket.  If the PT is bad you might get the 6.3 without tubes but some lower to no value when the tubes start drawing current.

[aro]

If it was me, I'd pull all the tubes and just measure 6.3VAC at the sockets.  If you get 6.3Vac, power-down, install the pre-amp tubes and measure the 6.3Vac in the PA tube sockets.  If it's still 6.3, power-down, install the PA tubes and measure on the bottom of a pre tube socket.  If the PT is bad you might get the 6.3 without tubes but some lower to no value when the tubes start drawing current.

I've done all that, and I have about 3.2VAC on each heater, with and without tubes. I apologize, I said earlier that heater voltages were in the mA range. I must have been on DC

Maybe I'll just have to replace the filter cap and that's it.
 

[labb]

You will have to measure heater voltage Pin to Pin. Not Pin to ground which is what it looks like you have done to get the 3.2 vac. Or you can measure each pin to ground and add the two.

[tubenit]

It sound like your PT is OK based on your responses about the high voltage at the rectifier and that the heater voltage is 6.4vac.

With respect, Tubenit

[aro]

I guess I dodged a bullet... Can't wait to change that filter cap.

[sluckey]

You will have to measure heater voltage Pin to Pin.

Yes. This issue comes up several times per year.

Or you can measure each pin to ground and add the two.

That's not reliable either. For example, you have a PT that has a 6.3VAC center-tapped filament winding. The CT is properly connected to ground. One filament lead is connected to pin 7 of a 6L6. But you forgot to connect the other filament lead to pin 2 or that wire is broken, or otherwise open. Connect your meter between chassis and pin 7 and you correctly read 3.15VAC. Move your meter probe to pin 2 and you still measure 3.15VAC. Add the readings and you think you get 6.3VAC but the tube will not lite up. Why? Because there is a broken circuit, no current is flowing thru the filament and the voltage you see on pin 2 is the same voltage you see on pin 7 because there is no voltage drop when there is no current flow. Some would be easily convinced that there's nothing wrong with the wiring. They might even start replacing tube sockets and tubes. Measuring voltage across the tube pins would have shown that there was really zero volts across the tube.

[aro]

Thanks for the explanation. I measured across to be sure and it is about 6.3V.

[aro]

Well, I got the new filter cap, replaced it, and same thing... Voltage in the mV range.

My amp is like this one: http://mhuss.com/MyJCM/JCM800_2204.gif , except that mine has only two filter caps. I don't have the one between V1a and V2a.  (Incorrect, it has three filter caps).

The filter cap with issues is the one next to the choke. Is it safe to assume that the choke is gone?

[shooter]

The filter cap with issues is the one next to the choke

does the one "downstream" read good? the shem shows  choke-50uF-10k2w-50uF.

One filter cap measured OK, the other one was in the mV range.

If this is still true, just leave out the "new" cap for now, since it appears you have B+ past the choke.
That tap/cap is used for G2 grids on your power tubes, if it's like the schematic.  Pull those tubes, add a cap back in and see if volts are good

[aro]

shooter, thanks. What I neglected to do properly was to look upstream from the choke. I totally ignored the obvious.

The voltage after the diode rectifier is OK. The first cap has solid voltage. Then comes the HT fuse. The voltage into the fuse is OK.

The voltages out of the fuse, into the choke, is in the mV's, so obviously the choke will have mV's going out.

Basically, the HT fuse is gone. I can't believe I didn't follow the circuit properly. Will see what happens after I replace that one.

[shooter]

totally ignored the obvious.

Easy enough to do, I think it's the best way to learn, sorta

[aro]

I remembered I had another 2204 around, so I used that fuse and now it's all good. No static, no noise. Phew... a lot of work... just to clear the cobwebs in my head. Now I understand the circuit again, like I did years ago before I stopped working with amps.

Now I can finally start working on the 18 Watt build that I never finished back then. Jeez, acquiring knowledge is a painful process... but fun.

[sluckey]

Just curious... Did you ever do anything with that layout I did for you several years ago?

[aro]

Just curious... Did you ever do anything with that layout I did for you several years ago?

It was about five years ago. I was literally shocked when I saw how old the files were on my computer. Five years went by... I went through some family events that didn't leave me any energy for amps, and very little for actually playing music.

Anyway, I am using your layout for my 18 Watt. I've got all the parts for the build and so far I've wired the heaters and some of the PT. I want to make one change though. I'll use 6V6 power tube instead of EL84's. (I know what I have to change in the power section to make it work with 6V6's)

This will be a high gain amp, and I don't like the El84 at high gain. Not a huge difference between them, but still... I played a Marshall DSL 15, which uses 6V6's, and I liked it better than another amp with very similar circuit and EL84's.

Thank you again for that layout. Even I can follow it.